The invention relates to a system for determining the angular spin position of a second object spinning about an axis, where a first object emits electromagnetic waves and where the system is provided with directional receiving antenna means fitted to the second object, and with a receiving system which, using the receiving antenna means, process in combination the carrier waves received to obtain said angular spin position.
Such an arrangement is known from EP-A No. 0,239,156, corresponding to U.S. Pat. No. 4,750,689. This patent particularly applies to a second object in the shape of a projectile. In case of fired projectiles, such as shells, it is often desirable to change the course during the flight. However, since a shell spins about its axis along the trajectory, correction of its course is effective only if at any random instant the associated spin or roll position .phi..sub.m (t) is well-known. Suitable course correction means for this purpose are preferably based on the principles of aerodynamics, chemistry, gas theory and dynamics. In this respect, considered are the deployment of damping fins or surfaces on the projectile's circumferential surface, the detonation of small charges on the projectile, and the ejection of a small mass of gas from the projectile.
According to the EP patent specification this problem is solved by transmitting signals consisting of at least two superimposed phase-locked and polarised carrier waves having different frequencies. These signals are transmitted by the first object.
Thus it is possible to obtain a reference signal by processing both carrier waves in combination. This reference signal comprises phase information of both carrier waves. By means of this reference signal, the 180.degree. uncertainty can be eliminated. It is clear from FIG. 1 of the EP patent that also a third carrier wave is present for transmitting data to the projectile by means of the transmitter. After this, for instance, the information on angle .phi..sub.g is transmitted upon which a correction is to be carried out by the projectile. For this purpose, the projectile itself determines the instantaneous angular spin position .phi..sub.m (t) and carries out a correction as soon as .phi..sub.g =.phi..sub.m (t).